One type of high speed data transmission network is defined by the Fiber Distributed Data Interface (FDDI) protocol. The FDDI protocol is an American National Standards Institute (ANSI) data transmission standard which applies to a 100 Mbps (Megabit per second) token ring network. The FDDI protocol is intended as a high performance interconnection between a number of computers as well as between the computers and their associated mass storage subsystems and other peripheral equipment.
Information is transmitted on an FDDI ring in frames (in FDDI-I) and cycles (in FDDI-II) that consist of 5-bit characters or "symbols", each symbol representing 4 data bits. Tokens are used to signify the right to transmit data between stations on the network. A "station" can include, for example, a computer (PC), a media access controller (MAC), station management (SMT) and a Physical layer device (PHY).
Of the thirty-two possible 5-bit symbol codes, 16 are data symbols (each representing four bits of ordinary binary data), 8 are control symbols, and 8 are violation symbols. The eight control symbols include J (the first symbol of a starting delimiter byte JK), K (the second symbol of the starting delimiter byte JK), I (Idle), H (Halt), Q (Quiet), T (End delimiter), S (Set), R (Reset), and L (Limiter). The violation symbols of the FDDI standard symbol set are not used because they violate code run length or DC balance requirements of the protocol.
Line states represent a long term condition of the link and therefore cannot be represented by a symbol or symbol pair. A continuous stream of control symbol patterns defines a line state. The FDDI protocol defines seven line states, including Idle Line State (ILS), Quiet Line State (QLS), Halt Line State (HLS), Master Line State (MLS), Reception of a start delimiter symbol pair JK (ALS), Noise Line State (NLS) and Line State Unknown (LSU). These line states are used to monitor and control the operation of the link. Line states are transferred between two stations to establish operational links, to monitor link quality and to provide for time-outs.
A repeater is sometimes required between a transmitting and receiving FDDI station. The repeater may be necessary to boost a signal which must be transmitted over a long physical distance between the transmitting and receiving stations. In another application, the repeater interconnects two segments of a path having different transmission media, such as optical fiber and copper.
In PHY devices previously used as repeaters, FDDI-I and FDDI-II standards are followed to filter the stream of 5-bit symbols that enters the repeater. Thus, when receiving a frame (i.e., operating in basic mode, i.e., FDDI-I mode), the PHY device will replace violation symbols with four Halt symbols followed by an Idle line state. This Idle line state is transmitted to a downstream receiving station until the repeater receives the next frame of data. When receiving cycles (i.e., operating in hybrid mode, i.e., FDDI-II mode), the PHY device will replace a violation symbol with an L symbol. When receiving neither frames nor cycles, violations are replaced with idle symbols.
The link errors are transferred into valid symbols and these errors are not detected and counted by the downstream receiving stations. Thus, the error count at the downstream station can be deceptively low, making the link between the upstream transmitting station and downstream receiving station appear to be in a better condition than it actually is. The link error processing is consequently impeded.
Another drawback of the conventional PHY repeaters is that they require software support from their associated station management (SMT) to process line state information. Upon detecting a line state from an upstream transmitting station, the repeater encodes the line state information and provides this encoded information to the SMT. Based on the particular line state received and other conditions on the FDDI system, the SMT software determines which line state should be transmitted. The SMT instructs a transmitter device within the repeater to generate this line state and send it to a downstream receiving station. The necessity for this software processing results in slower propagation of line state information. This lowers the limit on the number of repeaters that can be connected in series between two stations.
It is therefore desirable to have a PHY device which operates as an intelligent repeater which is able to repeat line state information without support from station management software. It is also desirable to have a PHY device which operates as a repeater which does not correct link errors. It is also desirable to have a PHY device which operates as an intelligent repeater that passes errors received to downstream receiving stations. It is also desirable to have a PHY device which can operate in both a conventional PHY repeater mode and in an intelligent repeater mode.